RFR (M) JDK-8150390: Move rs length sampling data to the sampling thread

[Mikael Gerdin]

Hi all,

The G1 Young RemSet Sampling Thread iterates over the young regions to 
estimate the remembered set sizes of the young gen in order to tweak the 
young gen size between gcs.

All the iteration and the state related to this has been stored in the 
YoungList class for some strange reason. The state is actually only ever 
needed in G1YoungRemSetSamplingThread::sample_young_list_rs_lengths()
so I've moved all of it there.
I also modified revise_young_list_target_length_if_necessary to receive 
the new rs length sample as a parameter instead of picking it up from 
the YoungList class.

Bug: https://bugs.openjdk.java.net/browse/JDK-8150390
Webrev: http://cr.openjdk.java.net/~mgerdin/8150390/webrev.0
Testing: Local GC test suite and JPRT

Thanks
/Mikael